[unreadable] The balance between epithelial cell growth and differentiation is likely to involve a complex interplay between classical cell regulatory pathways and development-related signals. The main working hypothesis of our presently funded grant is that Notch signaling plays an important role in control of keratinocyte self-renewal, differentiation and tumor genesis. We proposed to test this hypothesis with primary keratinocytes under well-defined culture conditions, by evaluating the consequences of increased Notchl activity, via adenoviral-mediated expression of the corresponding cDNA, or loss of the Notch-1 and/or Notch-2 "floxedgenes", via adenovirally-mediated expression of the Cre recombinase. We were further to test this hypothesis in vivo, by the analysis of mice with Cre-mediated keratinocyte-specific deletion of the Notchl and Notch2genes. We have found so far that increased Notchl activity is both required and sufficient for induction of mouse keratinocyte growth arrest, through increased p21 expression, and for selective increase of early versus late differentiation markers, through a mechanism distinct from that for p21 induction. We have further showed that Notchl functions as a keratinocyte tumor suppressor in both the mouse and human situation. Ongoing work is aimed at exploring the underlying mechanisms for Notch function in keratinocytes, by focusing on the interplay with other signaling pathways known to play a key role in these cells. We are here proposing to expand the scope of our work, to test the hypothesis that Notch signaling is involved in control of broad aspects of keratinocyte gene expression, that impinge on the self-renewal and differentiation potential of these cells. Our additional preliminary findings indicate that activated Notch 1 expression, unlike in mouse keratinocytes where it causes direct growth arrest, in human primary keratinocytes promotes the commitment of stem cells to transit amplifying but still proliferating populations. Thus, we will test whether by comparative global analysis of gene expression it is possible to identify genes with a key role in mediating Notch function in growth and stem cell potential of both mouse and human cells. [unreadable] [unreadable] [unreadable]